US2491938A - Method of producing viscose filaments - Google Patents

Description

NaOH IN VISCOSE- PER CENT Dec. 20, 1949 P. H. SCHLOSSER ETAL 2,491,938

METHOD OF PRODUCING VISCOSE FILAMENTS Filed Oct. 18, 1944 4' s e 1 a 9 IO 0 I2 13 CELLULOSE IN vrscose- PER CENT INVENTORS PAUL HENRY SCHLOSSER RElD LOGAN MlTCHELL KENNETH RUSSELL GRAY ATTORNEYS Patented Dec. 20, 1949 2,491,938 METHOD OF PRODUCING VISCOSE FILAMEN TS Paul Henry Schlosser, Reid Logan Mitchell, and

Kenneth Russell Gray,

Shelton, Wash., as-

signors to Rayonier Incorporated, Shelton, Wash, a corporation of Delaware Application October 18, 1944, Serial No. 559,234

9 Claims. 1

This invention relates to viscose production and has for its object the provision of an improved method of producing spun filaments and the like. More particularly, the invention provides an improved method of producing crinkled or potentially crinkled regenerated cellulosic filaments of viscose origin.

The method of the invention, in its broad aspects, is characterized by spinning a viscose solution into a coagulating and regenerating spin bath in which a slow regeneration is effected while the filaments are under low tension, regenerating completely while under low tension or in a relaxed state, and thereafter subjecting the filaments to high tension stretching while wet to develop such a condition in the filaments that the filaments crinkle spontaneously when wet with water under conditions permitting free shrinkage. The delayed stretching under high tension of the filaments which have undergone a slow or retarded regeneration results in a strained structure which causes them to crinkle.

The term low tension, as used herein, applies to operations in which the newly formed filaments are drawn oiT under low tension as in a regular spinning operation, for example, under tensions of around 0.3 gram per denier or less. In other words, the filaments, during the operation of coagulation and regeneration (partial at least), are under only sion that they are drawn through the spin bath at a speed in excess of the extrusion speed.

The term high tension stretching, asused herein, refers to tensions above 0.5 gram per denier and especially to tensions of 0.75 to 1.0 gram per denier or higher, or in any case sufficient tension to exert a maximum degree of stretching on the completely regenerated filaments. Actually the upper limit in high tension stretching is the point where damage to the filaments is noticed, either through visible breakage, or as evidenced by sticking of filaments on either stationary or roller guides.

In one of the embodiments of the invention, we use a viscose of special composition and spin the filaments in a coagulating and regenerating spin bath under low tension, and, after complete regeneration, subject the filaments to high tension stretching. The resulting regenerated and stretched cellulosic filaments have the capacity to crinkle spontaneously when treated with water under conditions permitting free shrinkage. The special viscose solution used in a method of the invention is relatively high in both the cellulose and the caustic soda contents and is spun a sufficiently positive tenit in an acid spin bath wherein the tension during the operation of coagulation and regeneration is very low; preferably only sufiicient tension is used to effect a positively controlled dinary non-crinkled yarns.

The single figure of the accompanying drawing illustrates compositions of the invention graph-- ically.

In carrying out our invention with viscose of special composition, we use a viscose solution having a cellulose content of from 9% to 13%, and a caustic soda content of from 8% to 13%, with a cellulose to caustic soda ratio of from 0.833 to 1.13. Under the most favorable conditions, a viscose containing about 10% each of cellulose and caustic soda gives the most effective crinkling. However, compositions close to this, such as 10 to 9, 10 to 11, 9 to 9, 9 to 10, and 11 to it of cellulose and caustic soda, respectively, give very effective and desirable crinkling under suitable spinning conditions. Graphically, the operative range of this invention may be expressed by the area encompassed by a pentagon bounded by the following points when located on the ordinate diagram of the drawings:

9% cellulose, 8% caustic soda 9% cellulose, 10.8% caustic soda. 10% cellulose, 12% caustic soda 13% cellulose, 13% caustic, soda 10% cellulose, 8.9% caustic soda values which have been found to be workable, namely, 9% to 13%. This preferred ratio of 1.0 becomes the more critical the more the value of 13% cellulose concentration is approached. In other words, the greatest leeway in the choice of the cellulose to caustic soda ratio Within the restricted ratio range of 1.13 to 0.833 is afforded at cellulose concentrations of not over 10% where the whole range is effective. But as the extreme high cellulose concentration is 13% is approached, the ratio of caustic soda to cellulose narrows down to the preferred value of 1.0. The viscose may have a salt index varying from around 4.0 to 7.5, a preferred range being from 5.5 to 6.0, and especially when spinning a viscose containing about each of cellulose and caustic soda.

In accordance with this aspect of the invention, a viscose solution of the special composition is extruded into or otherwise suitably contacted with a regenerating medium, for example, a normal Muller bath. A normal Muller bath, as referred to herein, is a spin bath in which sulfuric acid and sodium sulfate are present in such proportions that the bath consists substantially of a solution of NaHSOr together with some excess Na2SO4. Such a bath may contain in addition MgSO4, glucose and ZnSOr, the latter in amounts up to about 1.5%. The term normal Muller bath" does not refer to baths containing iron, manganese, chromium or other metals of that class in any appreciable quantity or zinc in amounts greater than 1.5%. Such special baths have a highly delayed regenerating action, and while sometimes used for spinning, nevertheless, are not of the character used in this embodiment of this invention employing a special viscose which has slow regenerating characteristics. The

. bath is preferably used at a temperature within the range of 45 C. to 55 C. However, some deviations from these upper and lower limits are permissible.

4 by a slow regenerating action. Spin baths which we have found suitable for the method of our invention compris compositions of about the following:

Grams per liter H2804 9O Na2SO4 300 ZnSO4 T 42 The filaments formed by extruding this special slow acting viscose into the normal Muller bath are drawn off under low tension as in a regular spinning operation on an ordinary spinning frame, and collected on one of the usual collecting devices. At this stage the filaments are normal in appearance and straight (and will stay straight if finished in a normal manner), but when they are stretched under high tension at a later time'and then allowed to shrink freely, crinkling occurs immediately and spontaneously. The filaments may be stretched after'complete regeneration while in the acid condition but preferably after washing. They may even be stretched after washing, drying and rewetting. The high tension stretching and crinkling will be described more fully hereinafter.

In another embodiment of the invention we use a normal viscose solution but spin the viscose under low tension into a coagulating regenerating bath of special composition which is characterized by a relatively slow regenerating action. The filaments spun under low tension are removed from the spin bath and permitted to age for a period in an acid condition until regeneration is complete and are thereafter subjected to high tension stretching with the result that the filaments crinkle spontaneously when wet under conditions permitting free shrinkage. The delayed or continuing regeneration following the low tension spinning in the spin bath may be effected in any suitable manner, preferably by collecting the filaments on a bobbin or in a cake in a more or less slack condition while Wet with adhering spin bath, washing in package form, and then subjecting them to the high tension stretching on a multiple end slasher.

In that embodiment of our invention in which we spin a normal viscose into a special spin bath under low tension, we effect a slow or delayed regeneration which is continued for a time beyond the spinning operation and before the high tension stretching. A salt index of about 4 is very suitable. By a normal viscose, we mean one containing from around 7% to 8.5% of cellulose and from around 6% to 7.4% of caustic soda, the ratio of cellulose to caustic soda being about 1.15. The special spin bath used in this adaptation of the invention is one which is characterized Specific gravity, 1.3.

After the filaments are subjected to coagulation and preliminary regeneration under low tension spinning, they are left in a more or less relaxed state for a short time, for example, from 10 minutes to from 3 to 4 hours, under regenerating conditions. This continuing regeneration may be effected while the filaments are 'wound on bobbins, in cake form, or in skeins and in contact with a regenerating medium, preferably in contact with adhering spin bath. The filaments should not be left in contact with the spin bath longer than necessary and particularly to a point where the acid begins to cause a deterioration of the cellulose. The filaments, after completion of the delayed regeneration, are ready for the high tension stretching.

The regenerated filaments, whether produced from the special viscose in a normal spin bath or from normal viscose in a special spin bath as previously described, are subjected to ahlgh degree of stretching in any appropriate manner. and at any convenient time, for example, while wet with the spin bath, or during or after washing, or during or after bleaching or even after drying and rewetting. The high tension stretching may be carried out as an operation entirely separate from the spinning operation, often conveniently during, or as a supplementary operation to, the finishing treatments. For' example, an operation such' as is used in the finishing of high tenacity tire cord yarn is very desirable. The filaments, if in cakes or on bobbins, may be stretched on a simple slasher, or, if in skeins, may be stretched in a skein stretcher at any convenient stage in the finishing operation (even after drying and rewetting) and then allowed to crinkle spontaneously under conditions permitting fre and rapid shrinkage. In any event, to secure the highest degree of crinkling, the regenerated filaments should be stretched in a wet condition to just below their breaking point, placed in water, if not already in water, and allowed to shrink as quickly and as freely as possible.

Stretching of the filaments in water may be to the extent of 30% to 50% but in case of rewetted filaments can be advantageously increased through the use of certain aqueous swelling baths as, for example, 1% NaOH in water or 5% dibenzyl dimethyl ammonium hydroxide in water.

The conditions which must be observed in the practice of the invention are somewhat critical. Slight variations in viscose composition, in spin bath composition, in bath temperature, in spinning tension and filament denier all affect the crinkle. Satisfactory crinkling depends upon the maintenance of a particular state during coagulation and regeneration, upon the acquiring of a latent crinkling structure, and upon the applying of stress to that structure.

At the completion of the high tension stretching, the filaments may be prevented from wrinkling either by drying them under tension or by chemically treating the wet yarn with a medium which inhibits crinkling. The result in either case is a straight product having the capacity to crinkle when treated with water under conditions permitting free shrinkage.

The simplest method of producing straight filaments with a capacity to crinkle is to simply dry the washed filaments under tension; for example by stretching and drying on a slasher. Another desirable procedure may be to lead the highly stretched and washed filaments while under tension through a bath containing, for example, a saturated solution of ammonium sulfate. If the spin bath is washed out of the yarn directly with the ammonium sulfate solution instead of with water, tension is not needed. In any case the filaments are prevented from crinklig by the inhibiting action of the ammonium sulfate and can be cut, opened and dropped-into water, thereby crinkling spontaneously, or may be preserved for long periods of time in the wet state as finished filaments having the capacity to crinkle.

A swelling bath, comprising, for example, 1% NaOH in water, or 5% dibenzyl dimethyl ammonium hydroxide in water, may be particularly useful in reswelling dried of crinkling.

Treatment of the stretched and straight filaments with water as by washing, dipping, spraying, or the like causes the strains in the stretched filaments to assert themselves and to crinkle the filaments when under conditions permitting free shrinkage. Wet filaments, stretched while still wet with spin bath, are usually not under conditions permitting free shrinkage due to the presence of substantial amounts of inhibiting electrolytes, such as sodium sulfate from thespinning solution. Treatment of such filaments with water reduces the concentration of the electrolyte, and in the absence of a mechanical restraining force, the filaments will crinkle. In the case of filaments washed and dried while restrained from shrinkage by mechanical force, treatment of the dry filaments with water, as by simple immersion, under conditions permitting free shrinkage, namely, in the absence of any restraining mechanical force on the filaments, causes the filaments to crinkle. Such shrinkage with water may be efiected with fresh water or with any suitable water solution providing the water does not contain a high concentration of a material that inhibits crinkling such as ammonium sulfate, sodium sulfate, or potassium fluoride. Only strong solutionsof these salts exert sufiicient lyotropic action to prevent crinklingeven a solution 'still allows good crinkling.

On the other hand as previously described, the water may contain materials which aid crinkling-as for example, small concentrations of NaOH or dibenzyl dimethyl ammonium hydroxide or a high concentration of KI or KCNS.

The following are illustrative examples of the invention:

Example I Viscose with a composition of 10% cellulose, 10% NaOH and viscosity of 50 sec. (falling ball) was spun at a salt index of 5.8 into a common spin bath (140 g./liter H280 245 g./liter Na2S04, 50 g./liter glucose, 12.5 g./1iter ZnSOr) maintained at 47 C. using 100-hole spinneret having holes 0.0020 of an inch in diameter. The collected filaments of 150 total denier were drawn out of the spin bath at a speed of 85 meters per minute and under a tension of about grams (less than .2 B./denier) and collected on a bobbin. The

filaments for purposes i The dried yarn was skeined l 6 bobbin of yarn was washed acid-free then put on a slasher, stretched 30% and dried under tension.

, then immersed in a hot water desulfuring bath containing about 1% NaOH; crinkling occurred immediately. The skeins were then bleached, oiled and dried. The filaments were highly crinkled, of dull lustre and skeins were bulky and of good hand. The tenacity was 2.3 g./denier dry, 1.3 g./denier wet; elongation (with crinkles straightened out) equalled 20% when dry and 30% when wet. The additional elongation due to crinkle was about If the yarn is collected in a centrifugal pot with a twist of 3 to 4 turns per inch, free shrinkage is somewhat restricted by the twist, but a bulky yarn is nevertheless obtained which shows a distinct spiral structure overlying the finer individual crinkles.

Example II Viscose made from wood pulp and having the composition of 10% cellulose and 10% sodium hydroxide at a sodium chloride index of 5.3 was spun into an acid spin bath with composition 125 g./liter H2SO4, 235 g./liter Na2SO4, 12.5 g./liter ZnSOi maintained at a temperature of 47.5" C. The viscose was extruded from a spinneret having 480 holes, each hole being 0.0030 inch in diameter at a rate to give filaments of 1100 total denier which were then drawn through the coagulating and regenerating bath for a distance of 100 inches under a spinning tension of 200 grams and then wound up on a bobin at a speed of meters/minute. Several bobbins of yarn were washed acid free and then on a simple slasher the collected ends were stretched 30% and led through a bath containing 50% (NH )2SO4 solution, excess solution being removed with squeeze roll. The bundle of filaments was then cut to 4-inch staples, opened and dropped into a large volume of water, thereby crinkling immediately and spontaneously. The 4-inch staple chips were only 1 inch in length after shrinkage; the takeup being largely due to development of crinkle. Each crinkled chip was very bulky and springy and could readily be drawn out into an elongated sliver merely by pulling with the fingers. Crinkles numbered about 25 to the inch and had an amplitude of about inch. Tensile tests on the single fiber gave a tenacity of from 2.5 to 3.0 g./denier.

Example III Viscose with a composition of 9% cellulose and 8% caustic soda, a viscosity of 60 seconds falling ball and a salt index of 5.4 was spun into a spin bath with a composition of 145 g./liter H2804, 330 g./liter NazSO4, 13 g./liter ZnSO4 and 55 g./liter glucose at 50 C. using a spinneret with 480 holes, each having a diameter of .0030 of an inch. The filaments of 1100 total denierwere grouped into a thread which was drawn from the coagulating and regenerating bath for a distance of inches under a tension of 200 grams at a speed of 85 meters per minute and collected on a bobbin. Staples cut from the bobbin at this stage and dropped into water showed no tendency to crinkle. However, when the yarn was stretched to just below the breaking point on two sets of rollers traveling at different speeds, then cut and dropped into water a fine crinkle developed immediately. It is important to subject the filaments to high stretch since the higher the 75 stretch the finer and more frequent the crinkles.

If, in the above example, a .viscose with the composition 'Z% cellulose and 6% caustic soda is used and the yarn subjected to the same spinning and stretching conditions no significant crinkling is obtained. If, on the other hand, a viscose with 10% each of cellulose and caustic soda is used, crinkle is somewhat finer than for a viscose with 9% cellulose and 8% caustic soda.

Example IV Viscose with a composition of 7.5% cellulose, 6.5% NaOH and viscosity of 35 sec. (falling ball) was spun at a salt index of 4.0 into a spin bath having a composition of 90 g./liter H2504, 300 g./liter NazsOi 42. g./liter ZnSO4 maintained at 47 C. using a 60-hole spinneret having holes 0.0031 of an inch in diameter. The collected filaments of 150 total denier were drawn vertically out of the spin bath after an immersion of inches at a speed of 85 meters per minute and under a tension of about grams (less than 0.2 g./denier) and collected on a bobbin. The yarn was washed acid free on the bobbin, skeined while wet, stretched and relaxed in water whereupon crinkling occurred immediately.

The crinkled skeins were then bleached, oiled and dried. The filaments were highly crinkled, of dull lustre and the skeins were bulky and of good hand.

In accordance. with the invention, filaments have been produced which show a wide variety of three-dimensional crinkle, both in filament yarn and in staple. The process is particularly adapted to spinning of fool-like staple fiber having filament deniers of 1-7.or even lower. The ability to spin crinkled filaments of such finedenier is particularly notable. Yarn of, for example, 2 denier per filament is exceptionally soft, has an exceedingly large number of crinkle per inch and has very good tensile properties. Tensile tests on single staple fiber show tenacities of greater than 3 g./denier dry with elongations of about 20% in the fiber itself not considering the elongation due to crinkle. The elongation due to crinkle in these staples is often greater than 100%.

From the preceding results and characteristics, it appears that the tendency to crinkle is an inherent characteristic of the filaments of the invention and after having once been stretched I and relaxed the crinkled form represents the normal equilibrium state.

When the straight dry yarn (washed, stretched and dried under tension) is treated with water while free to shrink, it is usually found that the crinkling incurred is somewhat less than the crinkling effect obtained with similar yarn which has been treated with water and allowed to shrink immediately after being taken from the stretching machine. In other words, washing and drying under tension tends to reduce somewhat the degree of crinkling'. It has, however, been found that with this dried yarn, certain solutions such as a aqueous solution of potassium iodide, a 1% solution of NaOI-I, or a 5% solution of dibenzyl dimethyl ammonium hydroxide return the yarn to a highly swollen gel state and give better crinkling than does water alone; no better, however, than is originally obtained if crinkling has been induced with water before the yarn has been dried at all.

Filaments preserved in the straight state with a substantially saturated solution of ammonium sulfate are not subject to marked deterioration and may be kept in the straight form, then converted at will into the crinkled form merely byv washing with water. Apparently, acid alone over a wide range in concentration has little, if any, effect on crinkling of the filament but will, of course, over a period of time if left in the filament, exert some degrading influence on the cellulose itself.

The different performance of the above-given examples of ammonium sulfate and potassium iodide when added in high concentrations to the wash water may possibly be explained by the progressive influence of the anions as exemplified in the well-known Hofmeister series registering the order of lyotropic action of ions. In this series the sulfate ion is near one extreme, whereas the iodide ion is near the other.

An experiment illustrating the permanency of the crinkles may be carried out by simply boiling some of the crinkled filaments in water for a period of several hours, then drying. Repeated treatments of this sort do not noticeably impair the crinkles. crinkles are so permanent in the wet state.

When a sample of crinkled filaments is dropped into a caustic soda solution of about 6% concentration, swelling is great and the crinkles disappear leaving the filaments essentially straight.

If, however, the filaments are removed from the caustic soda, washed with water free of caustic soda, and dried, it is found that the crinkles have been regained and to a degree comparable to the original. If a caustic soda solution of 18% concentration is used instead of 6%, the swelling is less and the crinkles do not completely disappear from the filaments while immersed in the caustic soda solution.

An important advantage of our invention is the providing of a simple, effective method for spinning crinkled or potentially crinkled viscose filaments on the conventional single bath type spinning frame commonly used for the spinning of non-crinkled yarns or non-crinkled staple fiber tow. For example, one type of single bath spinning operation to which the invention is particularly suited is carried out on a conventional viscose spinning apparatus which is not equipped with any stretching device whatsoever or in any case devices for applying high stretch. Such latter type of apparatus is commonly used for spinning of regular tenacity non-crinkled yarn at relatively low stretch. It is to be emphasized that the invention provides a method for spinning crinkled filaments in a single bath spinning operation without requiring any mechanical alterations from the apparatus normally used for spinning straight filaments.

The invention also obviously provides for a flexible method for changing a viscose rayon operation back and forth between production of crinkled and straight filaments without the necessity for mechanical alterations on the spinning frames.

We claim:

1. The improvement in the viscose process which comprises forming a viscose solution having about 10% each of cellulose and caustic soda, extruding the viscose into an aqueous acid spin bath to form cellulose filaments, said spin bath comprising about to grams per liter of sulfuric acid and 235 to 330 grams per liter of sodium sulfate in such proportions that the bath consists substantially of NaHSO4 with some excess Na2SO4 and around 12.5 grams per liter of zinc sulfate, coagulating and regenerating the fila- It is notable that the ments under a tension of less than 0.3 gram per denier, and then after regeneration is complete, subjecting the filaments to high tension stretching under a tension of at least 0.5 gram per denier while wet, whereby the filaments acquire the potential capacity to crinkle spontaneously when treated with water under conditions permitting free shrinkage.

2. The improvement in the viscose process which comprises forming a viscose solution having about each of cellulose and caustic soda, extruding the viscose into an aqueous sulfuric acid spin bath comprising about 125 to 145 grams per liter H2SO4, 235 to 330 grams per liter Na2SO4, and around 12.5 grams per liter ZnSO4 to form cellulose filaments and during the operation of coagulation and regeneration drawing the filaments from the spin bath under a tension of less than 0.3 gram per denier and then after regeneration is complete subjecting the filaments to high tension stretching under a tension of at least 0.5 gram per denier while wet, wherebythe filaments acquire the potential capacity to crinkle spontaneously when treated with water, and crinkling the filaments in water containing a swelling agent.

3. The improvement in the viscose process which comprises forming a. viscose solution hav-- ing from 9% to 13% of cellulose and from 8% to 13% of caustic soda, the ratio of cellulose to caustic soda being from 0.833 to 1.13, extruding the cellulose into an aqueous sulfuric acid spin bath comprising about 125 to 145 grams per liter H2504, 235 to 330 grams per liter NazSOs, and

12.5 grams per liter ZnSO; resulting in a relatively slow regeneration to form cellulose filaments, during the operation of coagulation and regeneration subjecting the filaments to a tension of less than 0.3 gram per denier, sufiicient only to eifect a positive withdrawal from the spin bath, and then subjecting the completely regenerated filaments to stretching under a tension of more than 0.5 gram per denier while wet, whereby the stretched filaments develop the capacity to crinkle spontaneously on wetting with water while free to shrink.

4. The improvement in the viscose process which comprises extruding a viscose solution into a coagulating and regenerating aqueous acid spin bath, drawing the resultant filaments from the spin bath at a rate at least equal to the extrusion rate under a tension not over 0.3 gram per denier to form straight cellulose filaments, removing the filaments from the spin bath and collecting them, leaving the collected filaments to relax and to relieve tension thereon while in contact with adhering spin bath for at least ten minutes, said spin bath. being of such composition in relation the amount of cellulose and caustic soda in viscose that regeneration is completed while relaxed filaments are in contact with the spin the the bath, and after regeneration is complete subiecting the straight filaments while wet to high tension stretching under a tension of more than 0.5 gram per denier, whereby the filaments develop the capacity to crinkle 5. The improvementkin the viscose process defined in claim 4 in which the viscose solution contains from 7% to 8.5% of cellulose and from 6% to 7.4% of caustic soda, the spin bath comprises approximately grams per liter sulfuric acid, 300 grams per liter sodium sulfate and 42 grams per liter zinc sulfate, and after the high tension stretching the filaments are wet with water containing a swelling agent to cause crinkling of them.

6. The improvement in the viscose process defined in claim 4 in which the viscose solution contains from 7% to 8.5% of cellulose and from 6% to 7.4% of caustic soda, the spin bath com prises approximately 90 grams per liter sulfuric acid, 300 grams per liter sodium sulfate and 42 grams per liter zinc sulfate, in which the filaments are dried after the high tension stretching while they are prevented from shrinking or crinkling, and in which the filaments thereafter are wet with water containing a swelling agent to cause crinkling of them.

7. The improvement in the viscose process definefl in 1claim74 in which the viscose solution con am-s rom to 8.5% of cellulose and r 6% to 7.4% of caustic soda. r em 8. The improvement in the viscose process defined in claim 4 in which the viscose solution contains about 7% cellulose and about 6% caustic soda, and the spin bath comprises approximately 90 grams per liter sulfuric acid, 300 grams per liter sodium sulfate, and 42 grams per liter zinc sulfate and has a specific gravity of about 1.3.

9. The improvement in the viscose process deiiined in claim 4 in which the viscose solution contains from 7% to 8% cellulose and from 6% to 7.4% caustic soda, and the spin bath comprises approximately 90 grams per liter sulfuric acid, 300 grams per liter sodium'sulfate, and 42 grams per liter zinc sulfate and has a specific gravity of about 1.3.

PAUL HENRY SCHLOSSER.

REID LOGAN MITCHELL.

TH RUSSELL GRAY.

REFERENCES CITED The following references are of record in th file of this patent: e

1 UNITED STATES PATENTS Number 7 Name Date 2,015,201 Stockly et a1. Sept. 24, 1935 2,233,418 Kline et al. Mar. 4, 1941 2,249,745 Charch et'al. July 22,, 1941 2,267,055 Tlppetts Dec. 23, 1941 2,287,839 I Stoeckly June 30, 1942 2,297,613 Fink et al. Sept. 29, 1942 1 2,297,746 Charch et al. Oct. 6, 1942 2,312,152 Davis Feb. 23, 1943 2,315,560 Stoeckly et al. Apr. 6, 1943 2,340,377 Graumann et al. Feb. 1, 1944 2,347,883 Cox May 2, 1944 2,347,884 Cox May 2, 1944 F REIQN PATENTS Number Country Date 462,184 Great Britain Mar. 4, 1937 483,404 Great Britain Apr. 20, .1938 492,279 Germany Sept. 19, 1922

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